The Spectrally Resolved Lyman-Alpha Emission Of Three Lyman-Alpha-Selected Field Galaxies At Z Similar To 2.4 From The HETDEX Pilot Survey

We present new results on the spectrally resolved Ly alpha emission of three Ly alpha-emitting field galaxies at z similar to 2.4 with high Lya equivalent width (> 100 angstrom) and Lya luminosity (similar to 10(43) erg s(-1)). At 120 km s(-1) (FWHM) spectral resolution, the prominent double-peaked Lya profile straddles the systemic velocity, where the velocity zero point is determined from spectroscopy of the galaxies' rest-frame optical nebular emission lines. The average velocity offset from systemic of the stronger redshifted emission component for our sample is 176 km s(-1) while the average total separation between the redshifted and main blueshifted emission components is 380 km s(-1). These measurements are a factor of similar to 2 smaller than for UV-continuum-selected galaxies that show Lya in emission with lower Lya equivalent widths. We compare our Lya spectra to the predicted line profiles of a spherical "expanding shell" Ly alpha radiative transfer grid that models large-scale galaxy outflows. Specifically, blueward of the systemic velocity where two galaxies show a weak, highly blueshifted (by similar to 1000 km s(-1)) tertiary emission peak, the model line profiles are a relatively poor representation of the observed spectra. Since the neutral gas column density has a dominant influence over the shape of the Lya line profile, we caution against equating the observed Lya velocity offset with a physical outflow velocity, especially at lower spectral resolution where the unresolved Lya velocity offset is a convoluted function of several degenerate parameters. Referring to rest-frame ultraviolet and optical Hubble Space Telescope imaging, we find that galaxy-galaxy interactions may play an important role in inducing a starburst that results in copious Lya emission as well as perturbing the gas distribution and velocity field, both of which have strong influence over the Lya emission line profile.